Apoptosis (programmed cell death) plays a central role in the development and homeostasis of all multi-cellular organisms. Apoptosis can be initiated within a cell from an external factor such as a chemokine (an extrinsic pathway) or via an intracellular event such as DNA damage (an intrinsic pathway). Alterations in apoptotic pathways have been implicated in many types of human pathologies, including developmental disorders, cancer, autoimmune diseases, as well as neuro-degenerative disorders. One mode of action of chemotherapeutic drugs is cell death via apoptosis.
Apoptosis is conserved across species and executed primarily by activated caspases, a family of cysteine proteases with aspartate specificity in their substrates. These cysteine containing aspartate specific proteases) (“caspases”) are produced in cells as catalytically inactive zymogens and are proteolytically processed to become active proteases during apoptosis. Once activated, effector caspases are responsible for proteolytic cleavage of a broad spectrum of cellular targets that ultimately lead to cell death. In normal surviving cells that have not received an apoptotic stimulus, most caspases remain inactive. If caspases are aberrantly activated, their proteolytic activity can be inhibited by a family of evolutionarily conserved proteins called IAPs (inhibitors of apoptosis proteins).
The IAP family of proteins suppresses apoptosis by preventing the activation of procaspases and inhibiting the enzymatic activity of mature caspases. Several distinct mammalian IAPs including XIAP, c-IAP1, c-IAP2, ML-IAP, NAIP (neuronal apoptosis inhibiting protein), Bruce, and survivin, have been identified, and they all exhibit anti-apoptotic activity in cell culture. IAPs were originally discovered in baculovirus by their functional ability to substitute for P35 protein, an anti-apoptotic gene. IAPs have been described in organisms ranging from Drosophila to human, and are known to be overexpressed in many human cancers. Generally speaking, IAPs comprise one to three Baculovirus LAP IAP repeat (BIR) domains, and most of them also possess a carboxyl-terminal RING finger motif. The BIR domain itself is a zinc binding domain of about 70 residues comprising 4 alpha-helices and 3 beta strands, with cysteine and histidine residues that coordinate the zinc ion. It is the BIR domain that is believed to cause the anti-apoptotic effect by inhibiting the caspases and thus inhibiting apoptosis. XIAP is expressed ubiquitously in most adult and fetal tissues. Overexpression of XIAP in tumor cells has been demonstrated to confer protection against a variety of pro-apoptotic stimuli and promotes resistance to chemotherapy. Consistent with this, a strong correlation between XIAP protein levels and survival has been demonstrated for patients with acute myelogenous leukemia. Down-regulation of XIAP expression by antisense oligonucleotides has been shown to sensitize tumor cells to death induced by a wide range of pro-apoptotic agents, both in vitro and in vivo. Smac/DIABLO-derived peptides have also been demonstrated to sensitize a number of different tumor cell lines to apoptosis induced by a variety of pro-apoptotic drugs.
In normal cells signaled to undergo apoptosis, however, the IAP-mediated inhibitory effect must be removed, a process at least in part performed by a mitochondrial protein named Smac (second mitochondrial activator of caspases). Smac (or, DIABLO), is synthesized as a precursor molecule of 239 amino acids; the N-terminal 55 residues serve as the mitochondria targeting sequence that is removed after import. The mature form of Smac contains 184 amino acids and behaves as an oligomer in solution. Smac and various fragments thereof have been proposed for use as targets for identification of therapeutic agents.
Smac is synthesized in the cytoplasm with an N-terminal mitochondrial targeting sequence that is proteolytically removed during maturation to the mature polypeptide and is then targeted to the inter-membrane space of mitochondria. At the time of apoptosis induction, Smac is released from mitochondria into the cytosol, together with cytochrome c, where it binds to IAPs, and enables caspase activation, therein eliminating the inhibitory effect of IAPs on apoptosis. Whereas cytochrome c induces multimerization of Apaf-1 to activate procaspase-9 and -3, Smac eliminates the inhibitory effect of multiple IAPs. Smac interacts with essentially all IAPs that have been examined to date including XIAP, c-IAP1, c-IAP2, and ML-IAP. Thus, Smac appears to be a master regulator of apoptosis in mammals.
It has been shown that Smac acts as an IAP antagonist promoting not only the proteolytic activation of procaspases, but also the enzymatic activity of mature caspase, both of which depend upon its ability to interact physically with IAPs. X-ray crystallography has shown that the first four amino acids (AVPI) of mature Smac bind to a portion of IAPs. This N-terminal sequence is essential for binding IAPs and blocking their anti-apoptotic effects.
The basic biology IAP antagonists suggest that they may complement or synergize other chemotherapeutic/anti-neoplastic agents and/or radiation. Chemotherapeutic/anti-neoplastic agents and radiation would be expected to induce apoptosis as a result of DNA damage and/or the disruption of cellular metabolism.
Current trends in cancer drug design focus on selective activation of apoptotic signaling pathways within tumors while sparing normal cells. The tumor specific properties of specific antitumor agents, such as TRAIL have been reported. The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is one of several members of the tumor necrosis factor (TNF) superfamily that induce apoptosis through the engagement of death receptors. TRAIL interacts with an unusually complex receptor system, which in humans comprises two death receptors and three decoy receptors. TRAIL has been used as an anti-cancer agent alone and in combination with other agents including chemotherapeutic drugs and ionizing radiation. TRAIL can initiate apoptosis in cells that overexpress the survival factors Bcl-2 and Bcl-XL, and may represent a treatment strategy for tumors that have acquired resistance to chemotherapeutic drugs. TRAIL binds its cognate receptors and activates the caspase cascade utilizing adapter molecules such as FADD. Currently, five TRAIL receptors have been identified. Two receptors TRAIL-R1 (DR4) and TRAIL-R2 (DR5) mediate apoptotic signaling, and three non-functional receptors, DcR1, DcR2, and osteoprotegerin (OPG) may act as decoy receptors. Agents that increase expression of DR4 and DR5 may exhibit synergistic anti-tumor activity when combined with TRAIL.
The beneficial effects of TRAIL production have been shown in several types of cancer. For example, intravesical instillation of the BCG vaccine induces a Th1 immune response, resulting in the production of anti-tumor cytokines, including TRAIL, and the infiltration of the lesion with immune cell and is the first line of therapy for the treatment of superficial bladder cancer. In vitro studies indicate that interferon alpha (INF-α), which in currently being tested in clinical studies for efficacy in bladder cancer, causes apoptosis mediated by the autocrine production of TRAIL in human bladder cancer cell lines. The circulating level of osteoprotogerin, a decoy receptor for TRAIL, is also increased in patients with bladder cancer and negatively correlate with tumor stage, grade and prognosis.
Moreover, it has been shown that TRAIL expression by NK (Natural Killer) cells is enhanced by IL-2 (Interleukin 2) treatment, and the expression of TRAIL is required for full tumor cell cytotoxic effects. IL-2, a cytokine, is currently approved for the treatment of both melanoma and renal cell carcinoma.
Inhibition of cancer cell replication and/or DNA damage repair will enhance nuclear DNA fragmentation, thus inducing the cell to enter the apoptotic pathway. Topoisomerases, a class of enzymes that reduce supercoiling in DNA by breaking and rejoining one or both strands of the DNA molecule, are vital to cellular processes, such as DNA replication and repair. Inhibition of this class of enzymes impairs the cells ability to replicate as well as to repair damaged DNA and activates the intrinsic apoptotic pathway.
The main pathways leading from topoisomerase-mediated DNA damage to cell death involve activation of caspases in the cytoplasm by proapoptotic molecules released from mitochondria, such as Smac. The engagement of these apoptotic effector pathways is tightly controlled by upstream regulatory pathways that respond to DNA lesions-induced by topoisomerase inhibitors in cells undergoing apoptosis. Initiation of cellular responses to DNA lesions-induced by topoisomerase inhibitors is ensured by protein kinases that bind to DNA breaks. These kinases (non-limiting examples of which include Akt, JNK and P38) commonly called “DNA sensors” mediate DNA repair, cell cycle arrest and/or apoptosis by phosphorylating a large number of substrates, including several downstream kinases.
Platinum chemotherapy drugs belong to a general group of DNA modifying agents. DNA modifying agents may be any highly reactive chemical compound that bonds with various nucleophilic groups in nucleic acids and proteins and cause mutagenic, carcinogenic, or cytotoxic effects. DNA modifying agents work by different mechanisms, disruption of DNA function and cell death; DNA damage/the formation of cross-bridges or bonds between atoms in the DNA; and induction of mispairing of the nucleotides leading to mutations, to achieve the same end result. Three non-limiting examples of platinum containing DNA modifying agents are cisplatin, carboplatin and oxaliplatin.
Cisplatin is believed to kill cancer cells by binding to DNA and interfering with its repair mechanism, eventually leading to cell death. Carboplatin and oxaliplatin are cisplatin derivatives that share the same mechanism of action. Highly reactive platinum complexes are formed intracellularly and inhibit DNA synthesis by covalently binding DNA molecules to form intrastrand and interstrand DNA crosslinks.
Non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to induce apoptosis in colorectal cells. NSAIDS appear to induce apoptosis via the release of Smac from the mitochondria (PNAS, Nov. 30, 2004, vol. 101:16897-16902). Therefore, the use of NSAIDs in combination with Smac mimetics would be expected to increase the activity each drug over the activity of either drug independently.
U.S. Pat. No. 6,992,063 to Shi et al. entitled “Compositions and method for Regulating Apoptosis” filed on Sep. 28, 2001 and issued on Jan. 31, 2006, herein incorporated by reference in its entirety, teaches that mimetics of the N terminal portion of Smac provide viable drug candidates.
Additionally, it has been shown in U.S. application Ser. No. 10/777,946 to McLendon et al. entitled “IAP-Binding Cargo Molecules and Peptidomimetics For Use In Diagnostic and Therapeutic Methods” filed on Feb. 12, 2004, herein incorporated by reference in its entirety, that a cargo molecule can be attached to a N-terminal Smac tetrapeptide peptidomimetic.